REJUVENESCENCE AND DEATH 303 



or capable of regression and rejuvenescence and whose develop- 

 ment is consequently most continuously progressive. In the higher 

 animals this organ is unquestionably the central nervous system. 

 This line of evidence, therefore, lends further support to the view- 

 that natural death is a death of the nervous system. 



In the warm-blooded vertebrates, where rejuvenescence plays 

 a minor part in the life history, the length of life in a particular 

 species is a more or less definite length of time, because the rate of 

 metabolism is largely independent of external conditions and the 

 rate of development and senescence is therefore determined largely 

 by internal factors which are more or less constant for the species. 

 In the cold-blooded animals, however, where rate of metabolism is 

 dependent on external temperature, senescence can unquestionably 

 be retarded, and so the length of life increased, by low temperature. 

 Moreover, in many of these animals long-continued starvation 

 and extensive reduction may occur with complete recovery, and 

 there is no doubt that under such conditions a greater or less degree 

 of rejuvenescence and consequently an increase in length of life 

 may occur in some cases. As regards the lower invertebrates, it 

 was shown in an earlier chapter that senescence may be retarded 

 or inhibited for a long time and probably indefinitely by the simple 

 means of underfeeding. This is of course not possible in the higher 

 animals, for their most stable tissues undergo senescence to some 

 extent even under these conditions. 



Among the lower animals and the plants cell death occurs, as 

 in the higher forms, as the end of progressive development, and 

 death of the many-celled individual may occur if progression and 

 senescence are not balanced by regression and rejuvenescence. 

 Even in the unicellular forms reproduction by fission brings about 

 some degree of rejuvenescence, and it is probable that nuclear and 

 cell division in general accomplish the same result to some slight 

 degree. When cells lose the capacity to divide they differentiate, 

 grow old, and sooner or later die. In short, the only conclusion 

 warranted by the facts is that death is everywhere the final result 

 of progressive development, if the process goes far enough, but in 

 many organisms progressive development is interrupted by regres- 

 sive processes connected with repair, reproduction, lack of food, or 



